1. Field of the Invention
The present invention relates to a control method for an injection molding machine, in which a servo valve is connected to a hydraulic cylinder for shifting a movable body, such as a movable platform, screw or so forth, for controlling a speed of the movable body.
2. Description of the Related Art
In general, in a clamping device Ms provided in an injection molding machine M as shown in FIG. 3, a servo valve 4 is connected to hydraulic cylinders 3x and 3y which drive a movable platform 2m (movable body 2) for shifting, for performing closed loop control of the speed of the movable platform 2m by supplying a control signal Cs derived on the basis of a difference between a speed detection value Vd and a speed command value Vc, from a controller 5.
On the other hand, since response of servo control is lowered when the servo valve 4 is employed, a measure has been taken conventionally in such a manner that an accumulator is connected to a discharge line of a hydraulic pump for maintaining stability and providing high response of the servo control by charging a high fluid pressure in the accumulator, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. Heisei 8(1996)-150642. When the accumulator is connected to the clamping device Ms shown in FIG. 3, it is typical to establish connection as illustrated by the phantom lines. In FIG. 3, a reference numeral 50 shown by phantom lines denotes an accumulator connected to a discharge line Ls of a hydraulic pump 6. Reference numeral 51 shown by phantom lines denotes a check valve connected between the hydraulic pump 6 and the accumulator 50 upon connection of the accumulator 50.
However, with such conventional control methods employing the servo valve, size and cost of the hydraulic circuit is increased due to the accumulator where an extra hydraulic pump is used for charging high fluid pressure to the accumulator. Also, since the accumulator has to be constantly charged with the high fluid pressure, power consumption becomes large, which in turn, increases problems with energy efficiency and economy.